Semiconductor integrated circuits are generally manufactured using a lithographic process. Since the minimum processing dimension of lithography depends on the wavelength of light used, it is necessary to shorten the wavelength of the irradiated light in order to improve the integration degree of the integrated circuit. Specifically, the lithographic process is, at present, performed using light having a wavelength of 157 to 365 nm. An object is to achieve the practical use of the lithography using extreme ultraviolet light having a wavelength of 11 to 14 nm.
For the light source of the extreme ultraviolet light, it is required that not only the light in the above wavelength region can be emitted, but also the light in the wavelength region can be selectively emitted (in other words, an emission in other wavelength regions is prevented), the light in the desired wavelength region can be emitted with high efficiency, and contaminating substances are not generated.
As a light source satisfying such conditions, a light source using a laser plasma method has been studied. According to the laser plasma method, a target is irradiated with a laser beam to form plasma, and extreme ultraviolet light emitted from the plasma is used. As the material of the target, materials in the form of gas, liquid, or solid, and made of various elements have been studied. Among them, as a gaseous target, one using xenon gas or the like is considered. A gaseous target is formed by injecting the gas into a predetermined region, and the gaseous target is used by irradiating the region with a laser beam. As a solid target, heavy metals or their compounds are considered. For example, a target made of a solid solution of 10% Th (thorium) in Sn (tin) matrix is disclosed in Japanese Unexamined Patent Publication No. 10-208998.
However, conventional targets have the following problem. The problem is described with reference to FIG. 1, in which the relation between a spatial position and an electron temperature in the vicinity of a surface of a conventional solid target is shown. Regarding a horizontal axis, the position of 1×104 μm corresponds to the position of a surface 15 of a solid target, and the position with a greater value in the horizontal axis corresponds to the inside of the target. The laser beam is irradiated toward the surface 15 of the target as shown by the arrow 13. In FIG. 1, the wavelength λL of the laser beam is 1064 nm and the light intensity IL is 1×1012 W/cm2. The region shown by the reference numeral 11 in the drawing is a region in which the formed plasma absorbs energy of the laser beam (laser absorption region), and the region shown by the reference numeral 12 is a region in which the formed plasma emits extreme ultraviolet light (extreme ultraviolet light emission region). When the target is irradiated with the laser beam, the energy of the laser beam is absorbed in the laser absorption region 11, the energy is transported from the laser absorption region 11 to the extreme ultraviolet light emission region 12, and the extreme ultraviolet light is emitted from the extreme ultraviolet light emission region 12 owing to the transported energy. Here, while the energy is transported from the laser absorption region 11 to the extreme ultraviolet light emission region 12, a loss of the energy occurs. That is, a part of the absorbed energy is converted to heat, or the ratio of light in a region other than a desired wavelength increases, so that the emission spectrum has a broader distribution. As a result, the emission efficiency decreases.
In addition to that, when the solid or liquid target is irradiated with a laser beam, a part of the target turns into particles without generating any plasma, and the particles disperse from the target. When the dispersed particles (debris) fall on an optical system or the like, or damages the optical system or the like, the precision of the system could be lowered.